A distributed antenna system (DAS) or a distributed radio system (DRS) generally refers to a radio-access architecture comprising a large number of antennas (or radio heads) distributed widely across a large coverage area and connected to a centralized access point (AP). The radiation coverage of each antenna typically has a much smaller footprint than that of a base-centrally-located antenna/base station in a conventional cellular system. The DAS architecture has two main advantages. First, it is possible to achieve high spatial re-use capacity due to the small coverage area of each antenna. Second, the centralized access point has complete control of all the radio resources used at each antenna and can therefore coordinate the transmission and reception of signals to minimize interference in an increased system capacity.
Typically, the antennas in a DAS are connected to the AP through optical fibers. The AP may process the received (uplink) signals from multiple devices using appropriate combining techniques, such as maximum ratio combing (MRC) or interference rejection combining (IRC). On the downlink, the AP may transmit to multiple devices using zero forcing or dirty papercoding to suppress interference if the forward link channel is known. The AP may also use macro diversity techniques to direct radiation to specific mobile devices if the channel is not known.
The capacity enhancements associated with DAS are well documented in isolated systems in which all antennas are connected to a single AP. In reality, the coverage area of an AP is limited by factors such as fiber length, propagation delay, and computing power. Therefore, a practical DAS more than likely will have multiple APs within respective coverage areas and mobile devices will need to be handed over from one AP to another in a manner similar to conventional cellular systems.
Handover may be approached in a DAS in a manner similar to a conventional cellular system. In a conventional system, the mobile device periodically makes signal strength quality measurements and reports the signal quality measurements to a radio network controller that coordinates handovers between a serving base station and a target base station. In general, a handover is triggered when the signal strength from a target base station exceeds the signal strength from the current serving base station by some predetermined amount.
The conventional method for handover can be problematic in a DAS. As previously described, the radio coverage area of an antenna in a DAS is much smaller than that of a centrally-located antenna in a conventional cellular system. Therefore, the overlap in coverage areas between two APs in a DAS is much smaller when compared to a conventional cellular system. For a mobile device traveling at moderate speed, the time in which to execute a handover will be much shorter than a conventional cellular system. This brief period may not be sufficient to complete a seamless handover, which may lead to a brief interruption in service. In a worst case, the handover may fail.